In building structures, it is often aesthetically desirable to cover large portions of the outside of the structures with as much glass as possible, and thereby provide a smooth and unbroken outside surface appearance. In order to provide such an appearance, it is necessary to minimize the appearance of the edges of the glass panels. Therefore, it is well known in the art to provide an adhesive bond beween the building structure and the inside surface of the glass panels which attaches the window panes to the building structure. Such bonding configurations are commonly known as "Structural Silicone Glazing," or "SSG" systems.
Typical SSG systems fall into two major classes; two-sided and four-sided. Four-sided SSG systems usually are comprised of a plurality of vertical structural mullions in combination with a plurality of horizontal structural mullions, which combine to form a mullion framework having a plurality of panel-shaped openings which are slightly smaller than the glass panels to be supported. The glass panels are positioned adjacent to the exterior surface of the mullion framework and over the panel-shaped openings by a plurality of temporary retaining clip such that the edges of the panels slightly overlap the panel-shaped openings and a small gap exists between the inside surface of the glass panels and the frame. Structural silicone adhesive is then applied in the gap. After the silicone adhesive cures, it provides a structural bond between the mullion framework and the glass panels which can completely support the glass panels without any aid from the temporary retaining clip or other outside retention means.
Additional silicone adhesive is then applied from the outside of the building into the gap created by the abutting edges of the glass panels, which provides a weatherproof seal. Disadvantageously, this "weatherbead" must be applied from the exterior of the building.
Two-sided SSG systems differ in that a structural bond is provided on the inside surface adjacent two opposing edges of the glass (usually the two vertical edges) and the corresponding frame member. In two-sided SSG systems, the two edges not being structurally bonded to the mullion framework must be retained by other means. This is normally done by conventional window glazing means which enclose the entire edge of the glass panel, which do not allow for the smooth, continuous appearance of the four-sided SSG system.
An improvement in adhesively glazed curtainwall systems includes prebonding an intermediate bracket to the inside of the glass panels, then allowing the structural silicone to cure, and thereafter mechanically fastening the intermediate bracket to the structural mullions. This resulting configuration, sometimes referred to as a "prebonded" SSG system, allows the structural silicone to be applied under controlled conditions, and insures more reliable and efficient adhesion of the glass panels to the building structure. However, the final silicone weatherproofing seal must still be applied at the pane edges from the exterior of the building.
Although such known prior art SSG systems are in demand, the cost for such systems is high. One reason is due to the fact that the final weatherproofing bead of silicone sealant applied to the gap between adjacent panels must be applied from the exterior of the building. This requires exterior scaffolding and relatively expensive field labor. Furthermore, the quality of the weather proofing joint is highly dependent upon the skill of the field laborer applying the sealant. Therefore, it is highly desirable to provide an improved SSG system which may be installed from the inside of the building structure during installation of the glass panels, which obviates the need for outside scaffolding and additional field labor to complete weatherproofing of the SSG window system.